Process of making electron discharge devices



- Patented Aug. 30, 1932- "UNITED. STATES PATENT OFFICE smemuNn 'LOEWE, or BERLIN, GERMANY, AssIoNon. To RADIO CORPORATION or AMERICA, A ooaronArIoN or DELAWARE PROCESS OF MAKING ELECTRON DISCHARGE DEVICES No Drawing. Application filed August 21, 1926, Serial No. 130,769, and in Germany August 25, 1925.

- This invention relates to aprocess of making vacuum tubes and more particularly vacuum tubes having highly emissive quallties. In accordance with this invention a vaporizable metal is dispersed in very small quantities in a highly evacuated tube, so that no mirror will be formed upon the bulb and I the transparency of the latter will be essentially preserved. According to this invention, furthermore, the-vaporizable metal, such alloyed with as magnesium, is preferabl the surface of a nonvappriza 'le body, which while being heated will give off only minute quantities of magnesium. A further feature of this invention consists in, that the vaporizable metal is made to form a part of the system of the tube, preferably the anode. Further features. of'this invention consist in the manner of alloying said nonvaporizable metal body with the "aporiza-ble metal.

According to the usual practice, in certain kinds of vacuum tubes highly emissive qualities are obtained by having, for instance, an ordinary tube with an incandescent cathode of thoriated tungsten first evacuated as fully as possible and thereupon subjected to a proc ess, which consists in the dispersion of a metal, such as for instance magnesium. Or-

' dinarily such tubes are rendered highly emissive after being subjected for some time to a process of formation or activation by keeping the cathode for some time in incandescent condition with or without applying the anode voltage thereto. The glass-bulbs of such tubes will seem from their exterior to have amirror inside the bulb, due to the dispersion or vaporization of the metal. This metallic mirror which is deposited on the interior surface of the glass-bulb results in a number of grave disadvantages.

In thefirst place it will be impossible or at least very diflicult, on account of the presence of such a mirror, to see whether the tube is in condition for operation, that is whether the cathode is in incandescent condition, and in the second place, the interior metallic mirror will reflect the heat to such a considerable extent that the entire heat carried off by radiation will bepassed through those parts of the bulb which are free of the mirror. Or-

perime'nts; furthermore a really stationary condition of operation will be brought about only after a very long time, that is until the temperature conditions have become stationary. Therefore, a continuous slowvariation M of the operating conditions will take place as long as the temperature conditions have not become stationary.

' This invention has for its object, to utilize all advantages which are connected with the dispersion or evaporation of the metal, and

at the same time avoid the above-mentloned disadvantages;

Experiments have shown that an extremely small vaporization of magnesium will be sufiicientto permit the formation or activation of the cathode to its full extent. It

appears that--in contra-distinction to the now prevailing opinion-the process of formation or activation is even favourably aflected by especially Weak metal deposits.

According to the known processes, therefore, unnecessarily large quantities of metal have-apparently been used for the process of dispersion, which on account of its useless surplus in the depositing of metalhas resulted in the above-mentioned disadvantageous features. However, in carrying out the process according to this invention, it is necessary to proceed in such a manner that the dispersion of the metal, although taking place only to a very small extent, is not started until a high vacuum has been obtained by carefully'degassing the metal parts of the tube.

It has further been found that it is advantageous to provide metal parts within the tube, such as for instance parts of nickel, which are superficially alloyed with ma mo nesium or another vaporizable metal. The

mode of operation, however, represents only one form of the 'rocess according to this invention. This a loy is made in such a manner that nickel, for instance in form of a sheet, is brought to incandescence in a vacuum furnace and that magnesium vapours are conveyed past the nickel body. The alloy which is thus obtained from nickel and magnesium has the property of again decomposing in the vacuum into its constituents. A peculiar feature has been found to exist with this process, viz., that the decomposing of the alloytakes place at temperatures which increase with the degree of vacuum and the temperature employed during the process of forming the alloy. In addition, during this decomposition the quantities of magnesium which. are again given ofi will be in inverse proportion to the degree of vacuum and the temperature employed during the process of forming the alloy. By properly choosing the conditions, therefore, one will be enabled to produce alloys which will again give 011' minute quantities of magnesium in the vacuum tube only when in a condition of incandescence or at very hi h temperature. Nickel-magnesium alloys w ich are thus produced are extremely Well suited for the purpose aimed at by this invention.

If for instance, the anode of a rectifying, amplifying or oscillating tube is made of a nickel sheet which is alloyed in the aforedescribed manner with difiicultly volatizable magnesium, a high temperature will be required in order to drive out minute quantities of magnesium from the body of the anode. These minute quantities of magnesium'will hardly have an eflfect upon the transparency of the glass-bulb and there will at most be a very slight blackening of the glass, but in no case is an opaque metallic mirror formed thereupon. The glass-wall of a tube which is made in this manner will still be almost as permeable for heat as a tube without mirror, whileexperiments have shown that the formation or activation of the cathode may be brought about in such case rather more easily than with tubes having the usual strong mirror.

The magnesium, which had been employed the glass-wall will in no Way obstruct the passage of Roentgen rays and also fully avoid the production of heat due to the passage of the rays through glass-walls having a heavy mirror.

I claim:

1. The method of producing an anode for an electron discharge tube which comprises shaping a sheet of metal to the form of an anode, heating said sheet to a temperature slightly higher than the temperature attained by said anode under normal operating conditions of the electron discharge tube, and exposing said sheet of metal while so heated to vaporized magnesium at a pressure no greater than the normal pressure in the electron discharge tube and thereby forming on said anode a coating of an alloy of said metal and magnesium which decomposes very.

slowly during normal operation of theelectron discharge tube.

2. The method of producing an anode for an electron discharge tube which comprises shaping a sheet of metal tothe formof an anode, exposing a sheet of nickel to vaporized magnesium at a pressure no greater than the pressure in the electron discharge tubeand heating said sheet while so exposed to a temperature slightly higher than the' normal operating temperature of said anode and thereby forming a surface alloy of nickel and magnesium which decomposes very slowly under normal operation of said anode in the electron discharge tube,

In testimony whereof I have aflixed my signature.

SIEGMUND LOEWE.

in the experiments, consisted of the ordinary chemically pure magnesium obtainable on the market, said magnesium having contained also admixtures of other substances, such as for instance, calcium.

It has further been found, that in the same a manner tungsten as well a's tantalum and molybdenum sheets may alloyed .with magnesium, so that one will be enabled to likewise produce electrodes having a. high melting point, which may be used'for highpowered tubes serving for transmitting purposes.

It is furthermore of especial importance, 

